1. Field of the Invention
This invention relates to a porous ceramic thin film having pores of uniform in diameter in the surface and a method for the production thereof.
Porous ceramic thin films are used in numerous products such as catalysts, catalyst supports, adsorbent materials, deodorants, deodorizing materials, and slow release materials.
2. Description of the Prior Art
As ceramic materials containing voids and pores, namely porous ceramic materials, there are known: 1) internal micropore-containing powders such as of zeolite and silica gel, 2) sintered materials such as of alumina ceramic and glass filter produced by sintering powders containing micropores, 3) porous glass produced by chemically treating or boring glass, 4) fibrous aggregates such as glass fibers and potassium titanate fibers, and 5) foams such as foam glass and cellular concrete [Hiroaki Yanagida: "Science of Ceramics", published by Gihodo, p. 109 (1981)]. These ceramic materials are invariably in a bulky state. Heretofore, it has been very difficult to produce a porous ceramic film, particularly a porous thin film. It has been virtually impossible to produce a porous thin film containing pores of a uniform diameter. Such methods as PVD, CVD, and sputtering and a method for anodic oxidation of metals have been available for the production of porous ceramic materials. These methods are at a disadvantage in being expensive to implement, incurring difficulty in obtaining products with large surface areas, encountering difficulty in controlling micropores, and limiting the metals usable as raw materials and consequently limiting the ceramic materials produced.
If porous ceramic thin films containing micropores of a uniform desired diameter can be easily produced, it will be possible to use them for efficient adsorption of substances that have offensive odors or pollute the environment. This can be achieved by adjusting the size of the micropores of the porous ceramic thin film to conform to the size of the particles of the substance to be adsorbed.
For the removal of a substance that possesses an offensive odor or pollutes the environment, the practice has been to absorb or adsorb the substance using a liquid absorbent such as an acid or an alkali, an adsorbent agent, or soil. This method, however, requires disposal of a liquid waste or the used adsorbent agent or soil and, therefore, not only is troublesome but may also cause secondary environmental pollution. A method which masks offensive odors with an aromatic and a method which decomposes malodorous substances with activated sludge or ozone are also conceivable. The method using an aromatic has the problem that the smell of the aromatic itself may cause pollution, the method using activated sludge has the problems of low treatment capacity and inevitable diffusion of the smell of sludge, and the method using ozone has problems deriving from the harmful nature and high cost of ozone [Konosuke Nishida: "Encyclopedia", published by Heibonsha, Vol. 1, p. 136 (1984)].
It is known that when ceramic materials are semiconductors, they manifest special effects. Specifically, when a ceramic semiconductor is irradiated with light, it generates electrons having a strong reducing activity and positive holes having a strong oxidizing activity and causes molecules which contact it to be decomposed by the oxidizing-reducing activity. By harnessing this photocatalytic activity of the ceramic semiconductor, organic solvents dissolved in water, such substances as agricultural pesticides and surfactants which pollute the environment, and noxious substances in the air can be removed by decomposition. This method utilizes exclusively a ceramic semiconductor and light. It has an advantage over biotic treatment methods using microorganisms in that it is less restricted by such reaction conditions as temperature, pH, gaseous atmosphere, and toxicity. Moreover, it enables organic halogen compounds and organic phosphorus compounds, which are not easily treated by biotic methods, to be easily decomposed and removed. The studies performed to date regarding the removal of organic substances by photocatalytic decomposition have used photocatalysts in the form of powder [as reported in A. L. Pruden and D. F. Ollis, Journal of Catalysis, Vol. 82, 404 (1983), H. Hidaka, H. Jou, K. Nohara, J. Zhao, Chemosphere, Vol. 25, 1589 (1992), Teruaki Hisanaga, Kenji Harada, and Keiichi Tanaka, Industrial Water, Vol. 379, 11 (1990), for example]. These photocatalysts are difficult to handle or use. In the treatment of water, the treated water must be filtered to recover the photocatalyst powder entrained therein. Since the photocatalyst is a fine powder, it is not easily separated by filtration because it clogs the filter. The treatment of water, therefore, cannot be carried out continuously because the recovery of the used photocatalyst from the treated water is difficult. The development of a porous thin film, i.e. a photocatalyst (ceramic semiconductor), which is easy to manufacture will enable continuous adsorption and decomposition of harmful substances to be conducted and efficiently on a maintenance-free basis.
Among the various kinds of porous ceramic thin films, there is a particularly strong need for one having micropores of uniform diameter. Further, a need has arisen for a photocatalyst that is free from the problems mentioned above. The present inventors continued a study with a view to meeting these needs. This invention has been accomplished as a result.